1. Field of the Invention
The present invention relates generally to echo canceller systems in communication networks. More particularly, the present invention relates to methods and systems for fast reconvergence of echo cancellers after TDM slips and echo level changes.
2. Background Art
Subscribers use speech quality as the benchmark for assessing the overall quality of a telephone network. A key technology to provide a high quality speech is echo cancellation. Echo canceller performance in a telephone network, either a TDM or packet telephony network, has a substantial impact on the overall voice quality. An effective removal of hybrid and acoustic echo inherent in telephone networks is a key to maintaining and improving perceived voice quality during a call.
Echoes occur in telephone networks due to impedance mismatches of network elements and acoustical coupling within telephone handsets. Hybrid echo is the primary source of echo generated from the public-switched telephone network (PSTN). As shown in FIG. 1, hybrid echo 110 is created by a hybrid, which converts a four-wire physical interface into a two-wire physical interface. The hybrid reflects electrical energy back to the speaker from the four-wire physical interface. Acoustic echo, on the other hand, is generated by analog and digital telephones, with the degree of echo related to the type and quality of such telephones. As shown in FIG. 1, acoustic echo 120 is created by a voice coupling between the earpiece and microphone in the telephones, where sound from the speaker is picked by the microphone, for example, by bouncing off the walls, windows, and the like. The result of this reflection is the creation of multi-path echo, which would be heard by the speaker unless eliminated.
As shown in FIG. 1, in modern telephone networks, echo canceller 140 is typically positioned between hybrid 130 and network 150. Generally speaking, echo cancellation process involves two steps. First, as the call is set up, echo canceller 140 employs a digital adaptive filter to adapt to the far-end signal and create a model based on the far-end signal before passing through hybrid 130. After the local-end signal, including near-end signal and/or echo signal, passes through hybrid 130, echo canceller 140 subtracts the far-end model from the local-end signal to cancel hybrid echo and generate an error signal. Although this echo cancellation process removes a substantial amount of the echo, non-linear components of the echo may still remain. To cancel non-linear components of the echo, the second step of the echo cancellation process utilizes a non-linear processor (NLP) to eliminate the remaining or residual echo by attenuating the signal below the noise floor.
Due to changes in the echo path, echo cancellers may restart the adaptation process to readjust the echo cancellation parameters. Echo path changes may occur due to a variety of reasons such as when there is a clock slip in the Time Division Multiplexed (TDM) bus that carries the coded speech, such as G.711 coded speech, or when there is a change in the echo level. As a result, conventional echo cancellers restart the adaptation process when there is a clock slip in the TDM bus and/or when there is a change in the echo level. Such restart process is known to be time consuming and even more, quite undesirable, because while the adaptive filter goes through the re-adaptation process, the echo signal is not being cancelled effectively.
Accordingly, there is a need in the art for echo canceller systems that can converge or adapt quickly when there is a clock slip in the TDM bus and/or when there is a change in the echo level, without restarting the adaptation process.